1. Field of the Invention
(A1) The present invention relates to a differential code transmission system, more particularly, different from the conventional system of this kind, such that samples of an input analog signal such as an audio signal are separated, for instance, every other one into two groups of odd numbers and even numbers, in each of these two groups respectively, successive differences of samples being coded, so as to be transmitted or recorded.
(B1) Moreover, it relates to a system of this kind, more particularly, successive differences of samples being near instantaneously compressed, so as to be accurately transmitted.
(C1) Furthermore, it relates to a system of this kind, more particularly, correctable without any correction code added, even if errors are caused during the transmission or the recording.
(D1) Generally, it relates to a system of this kind, more particularly, to the code transmission effected through the code conversion from 3 bits of a binary code to 2 symbols of a tertiary code, with a bit error rate reduced.
2. Related Art Statement
(A2) According to the conventional differential code transmission system, in a differential coding apparatus, continuously successive samples of an input analog signal are coded and then transmitted or recorded. So that, the higher the frequency of the signal is raised, the narrower the dynamic range of the apparatus is reduced, and hence the distortion of the coded signal having ample components in the higher frequency range is extremely increased as the case may be. This situation may be readily understood by referring to FIGS. 1a and 1b showing the relations between the frequency and the amplitude of differential components of the signal in comparison between the case as shown in FIG. 1a that the conventional differential PCM coding is applied on the low frequency signal and the other case as shown in FIG. 1b that it is applied on the high frequency signal. In this connection, throughout FIGS. 1a and 1b, a segmental line capped with a circular mark indicates a uniformly quantized signal, while another segmental line capped with an arrow indicates a differential component obtained by the conventional coding system.
(B2) Moreover, as for the conventional technique, an error correction effected by using a local decoder is frequently carried out in the conventional differential PCM coding apparatus, while no differential coding apparatus effecting near instantaneous compression and expansion has not yet disclosed. As a documental example concerned, "PCM Transmission System" edited by the Japanese Eelctronication Society, 1974, pp.242 relating to the differential PCM apparatus for transmitting the picture signal can be found.
On the other hand, Japanese Patent Laid-open Publication No. 63-16,718, "Near Instantaneous Compression Coding System" written by the present inventors discloses a near instantaneous compression coding system in which successive differences between adjacent samples of an input analog signal are applied with near instantaneous compression through a range compressor controlled by a range signal indicating a range compression factor set up in each of predetermined sections dividing successive samples of the input analog signal in response to the maximum value of the difference between successive samples in each of the sections and thereafter an output of the range compressor is transmitted together with the range signal concerned, featured in that the difference of successive samples is obtained between an immediately preceding sample derived from a local decoder provided on a sending side and a present sample, while an error caused by the range compression in the signal to be transmitted is corrected through a feedback arrangement.
In addition, in the conventional differential PCM transmission system, the difference between successive samples in the lower frequency range is small and hence can be transmitted with a smaller number of bit, while the difference between successive samples in the higher frequency range becomes large, so that the reduction of transmitted bits cannot be attained by the differential coding.
In connection with the above, the transmission efficiency has been conventionally increased by compressing the differential signal on the sending side, while by expanding it on the receiving side. However, it is impossible to neglect the received signal distortion based on the errors caused in the lower frequency range by the reduction of the number of transmitted bits which is based on the above compression of the differential signal to be transmitted.
The aforesaid near instantaneous compression coding system disclosed in the Japanese laid-open publication has been conceived, so as to prevent the above defect of the reduction of the number of transmitted bits by effecting the above described error correction in the near instantaneous compression and expansion. However, even this near instantaneous compression coding system is not yet sufficient.
(C2) Furthermore, the most significant bit of the differential coding (DPCM) signal contains an information corresponding to the sign indicating the positive (increasing) and the negative (decreasing) differential coefficient of the original analog signal at the sample point, unlike that of the linear pulse coding (PCM) signal at the sample point thereof. However, any conventional technique for utilizing this information contained in the MSB as for the erroneous code correction has not yet disclosed.
In addition, the DPCM signal has a peculiar property such that an error once caused during the transmission or the recording is propagated to consecutive coded signals, unlike the linear PCM signal. More particularly, the MSB signal indicates the increase or the decrease of the difference between successive samples, so that the erroneous MSB signal has a serious influence upon the decoded signal. For avoiding this serious influence, it may be conceivable to apply an intense correction code particularly upon the MSB signal. However, it is frequently inappropriate in case that the capacity for the recording or the transmission is limited.
(D2) Generally, in the transmission of the binary code, it is frequently required for restricting the frequency band width of the transmitted signal to convert the binary code into a certain multi-level code. However, the employment of the multi-level code inevitably complies the reduction of threshold levels for discriminating the received multi-level codes and hence the code error of the multi-level code received through a noisy transmission path is frequently caused by the noise. As a result, in the counter conversion of the multi-level code into the binary code, the extent of bit error caused in the converted binary code is varied in response to the coding method employed.